Secondary-electron emission



Oct. 3l, 1950 J, BRAMLEY 2,527,981

SECONDARY-ELECTRON-EMISSION Filed April 9, 1948 3 Sheets-Sheet 1 INVENTOR oct. 31, 195o J. BRAMLEY 2,527,981

SECONDARY-ELECTRON-EMISSION 3 Sheets-Sheet 2 Filed April 9, 1948 75 Z5 Aw Oct. 3l, 1950 J. BRAMLEY 2,527,981

SECONDARY-ELECTRoN-Emssrou Filed April 9, 194:3l s sheetssheet s INVENTOR Y Jew 311m@ www ATTORNEYS Patented Oct. 31, 1950 SECN-DARY-E-ALECTRON EMISSION .lenny Bramley, Long Branch, N. J.

Original application. AugustV 23,. `194:5, Serial No.

612,197. Divided and this 1948, Serial N0.f20,016

24 Claims. l

My invention relates to apparatus for secondary-electron-emission.

The present application is a division of my copending application for Secondary-Electron Emitting Surface, Serial No. 612,197, led` August 23,1945.

The primary purpose of the invention is to obtain: multiplication of secondary electrons much higher tha-n that obtained in prior practice. This is vaccomplished in this invention by the proper construction of composite surfaces. One method is to interpose a layer of dielectric limited in thickness to 0.1 mm. and preferably not exceeding 0.03 mm. between e. layer with at least moderately high secondary-electron-emissive properties (called henceforththe secondary-electronemitting layer) on the onehand and a metallic baseon the other hand. Suitable substances for the dielectric are oxides ofY titanium, coppen.

manganese, or aluminum, theV latter having impurity centers, or crystals of alkali halides. .For

the secondary-eleetron-emitting layer the follow- A further purpose is to produce a secondaryelectron-emitter ofhigh multiplication while avoiding time lag between the start or stop ofthe primary current andthe start or stop of high secondary electron emission.

A further purpose is to maintain close vtim coordination between asecondary-electron-emitter and the primary current by providing for the neutralization by electronsof the positive charge in the secondary-electron-emitting layer at a rate predetermined by the primar-y current.

A further purpose is to enhance the secondaryelectron-emission of magnesium (oxidized) by baking it in vacuo at a temperature of 600 to 800 C. Other secondary electron emitters should be baked at appropriate temperatures predetermined by the nature of the emitter (that is, from r 60 C. to a temperature of 300 C. below the melting point) in order to obtain close coordination between the primary current Aand the secondary `electron emission together with thel corresponding optimum value for the secondary electron application April 9,

(Cl. 25W-174) emission ratio. It is possible either to minlmi'ze the time lag between the primary and secondary currents'or to obtain the maximum value for the coefficient of secondary emission, but in general it is not possible to accomplish' both purposes simultaneously. In the case of the alkaliV hali'cles this temperatureshould range from 60 C. up to 250 C. in order to control the speed of the color centers induced in the alkali halides by primary electron bombardment, when these color centers move under the inuence of an electric field.

A further purpose is to oxi'diZe a metallic base, such as aluminum, copper or chromium, or an alloy thereof, or a corrosion resisting ironchromium alloy (such as stainless steel), forming a dielectric, 'apply over it a secondary-elece tron-emitting layer, and expose the layers duralong the layer of the secondary-electron-emitter in the composite surface as by granulating the surface into a mosaic.

A further purpose isto increase the' scope of cathode ray beam tubes by employing a fine Wire mesh with a surface having' enhanced" secondary-electron-emitting' properties in a cath-l ode ray beam tube in which' there is' a partially transparent metal lm on the inside oi' the face plate. Aluminum constitutes a suitable' choice for the metal film. The secondary-electronemitting surface should surround the wires of the mesh, the whole structurev constituting a perforated surface, which not only will allow the primary beam to go through it, but in' its travel infascanning path to iind at all points a channel through which electrons can pass. The mesh must be at a negative potential with respect to the thin metal on the face plate; In another embodiment the fine wire mesh is'u's'ed to support the three-layer secondary electron emitter,

which should approximate a continuous surface.

The base metal of the three-layer surface should in a variation of this embodiment make electrical Contact with the fine metallic wire mesh used to support the secondary electron emitter. In another varation of this embodiment, the line metallic wire mesh can be used to supply the heat to control the temperature of the three-layer emitter. There is still another variation of this embodiment in which the three-layer secondary electron emitter can be supported in a non-metallic grid structure such as plastic or glass. In still another variation the aforementioned grid is replaced by a thin transparent sheet of plastic or glass. In those cases in which the dielectric of the composite secondary electron emitter has the optical properties desired for the examination of the pattern produced by the electron bombardment, the base metal should be in the form of a thin partially transparent film.

By proper choice of the emitting surface, the time lag between the primary current, andthe secondary` emission from the wire mesh can beY made comparable to the time between scanning frames. This is equivalent to a storage effect. The drawings are useful in explaining the invention. They have been chosen for the purk pose of clear illustration of the principles involved and to illustrate conventionally a few only of the possible embodiments of the invention. In the drawings like numerals refer to like parts.

4Figure l is a diagrammatic section of a composite surface useful in explaining the invention.

Figures 2, 6, 7, 8 and 10 show cathode ray beam tubes having a glass envelope, the conventional electron gun and electrode system for accelerating the electrons, and if necessary focusing and deiiecting them, being omitted in Figures 2, 6, 7 and 10, and suggested in Figure 8.

' Figure 3 is a diagrammatic section of a composite surface in which the metal base makes electrical contact with a fine metallic wire mesh. Figure 4 is a variation of Figure 3 in which the vwire mesh supplies heat.

Figure 5 is a variation of Figure 3 in the mesh is of plastic or glass. K Figures 6 and 7 are variations of Figure 2 omitting the grid and applying the secondary ,electron emitting element to the face plate.

Figure 8 is a further variation of Figure 2.

Figure 9 is a fragmentary variation of Figure 8.

Figure 10 is a view corresponding to Figure 2 Ashowing a further variation.

One of the most important aspects of the invention relates to the production in electron ytubes of extremely high emission of secondary `electrons as a consequence of strong electrolstatic fields initiated by bombardment with primary electrons of composite surfaces constructed Ain accordancewith the invention. There must, `of course, be a suitable source of primary electrons and these electrons should have a speed such that the ratiov of the number of secondary electrons released from the secondary-electron- `emitting element to the number of primary electrons of the beam impinging on the secondarywhich 'electron-emitting element is greater than unity,

'such velocity being furthermore such for cathode 'ray tubes that the time lag between the impact .of the electrons in the scanning beam and the emission of secondary electrons is less than the time of persistence of vision.

The base, dielectric and secondary emitter are vcapable of being laid variously but always with `the same resultant arrangement of layers and the same method of operation.

In the construction of tubes, the electrodes 4 actually formed within the envelope itself by applying one part at a time until the construction under consideration has been completed, or the features or elemental parts may be correlated outside of the envelope with more or less completeness and be then introduced within the envelope as a member of the assembly, after which the tube is closed and evacuated.

The metallic base in Figure 1 may consist of any desired solid metallic material. The metallic base is covered by an extremely thin layer 2| of dielectric. Suitable dielectric materials are: aluminum oxide A1203, provided it contains impurity centers, titanium oxide T102. copper oxide, manganese oxide MnO, inorganic Y crystalline phosphors, inorganic vitreous phosphors, or an alkali halide, such as NaCl, KCl, LiCl. The alkali halides are inert and do not volatilize easily under the conditions of production of secondary emission.

The dielectric need not be a perfect insulator; it preferably will be a semi-conductor. The structure of the dielectric should preferably be very fine.

On the dielectric layer is deposited a thin (2 to 20 microns thick) layer 22 of a substance which is a good emitter of secondary electrons such as beryllium; beryllium oxide, BeO; alloys of beryllium, particularly alloys of berylium and copper; magnesium (oxidized) oxidized magnesium aloys; aluminum; alloys predominantly aluminum, such as duralumin of any of the recognized varieties, especially aluminum alloy 17S (Al 95%, Cu 4%, Mg. 0.5%, Mn 0.5%) 0r aluminumalloy 24S (Al 93.8%, Cu 4.2%, Mg 1.5%, Mn 0.5%), or the aluminum magnesium (such as 10% or 30%) alloys, or the aluminumberyllium alloys, of which the one containing 30% beryllium appears the most efficient. Aluminum base alloys of silicon, and aluminum base alloys of copper may be used. In the aluminum base alloys of beryllium the content of beryllium should be between 25% and 40% and the combined content of magnesium, molybdenum, and zirconium should not exceed two percent; oxides of aluminum base alloys of beryllium may be used. Also oxidized aluminum base alloys of beryllium having magnesium as a preconstituent may be employed. Among the above, the duralumins are unexpectedly effective out of all proportion to any characteristics previously suspected and greatly exceed pure aluminum in multiplication.

When this top or secondary-electron-emitting layer 22 is struck by primary electrons under suitable potential conditions, it remits a large number of secondary electrons and thus becomes positively charged and creates a strong electrostatic field between the metallic base and the dielectric layer 2|. This electrostatic field pulls out electrons from the metallic base 2l) through the dielectric thus producing a high multiplication of electrons. The strong fields across the dielectric which have been discussed so far, were produced by secondary electron emission from the secondary-electron-emitting layer. However, if the secondary-electron-emitting layer is conducting, it is possible to connect it electrically to a source 'of electrical power and obtain the same results, namely, the production of internal secondaries in the dielectric by the primary beam, the production of electrons in the dielectric by strong eld emission from the metal base, and in certain cases, such as alkali halide dielectrics, the drift of color centers.

v -Thec'ombined=fthickness of Lthe dielectric layer and of the ulayer l`emitting secondary electrons mustbe very-smalL the-desirable range being from 2 to 20 microns. While not in every -case essential, this range of--lthicklness should' be f used for bestresults. Y `Y 'One nofllthe problems in the prior art has been to 4cause the secondary-electronemission to -stop and start either -incoincidence-with the primary current or after only a -brief and controllable time interval. 'Malter fMarconi) British Patent 481,-1'70 ,1September"7,' 1936,Y uses ecaesium and Ais troubled byftime lag between the startvof vthe primary'currentand the sta-rt othehighsecOndaryelectron-emission, asJwel-lasbetween-the-stopping of the primary A.current Aandthe stopping 4of the lsecondary emission. (Malten Physical Review, vol. 49, Jp. V478 and 4p 8'79` ("193`6) Furthermore, vcaesiuml deteriorates by volatilization V`in rvacuo and causes objectionable Aphotoe'lectric effects, which militate against the Auseiof the layer las van emitting surface-in a photomultiplier tube.

In the present 'inventionfthe electron-emitting `layexris non-photoelectric, and many advantages and avoidance of much1difliculty'arefthereby obtained.

In order to Yprevent excessive time lag, the `positive charge kin 'the secondary-electron-emitting `layer I22 must 'be' neutralized by electrons from the dielectric 2| very quickly, but not quickly enough Y'to interfere with extraction of secondary electrons by 'the electrostatic eld. The -eXtractiontime has been estimated as about l0-14 seconds.

The Vdielectric layer 21 may be deposited by centimeter) the thickness 'of the dielectric layer is of importance. .A voltage due 'to Ysecondaryelectron emission of more than a few thousand volts 'is not obtained in practice. For best results Vthe thickness should 'be approximately 0.03 millimeter, and in any case the thickness of the layer should notexceed 0.1 millimeter. yNo vlimit yon thinness is necessary provided the dielectric functions.

V'I 'he invention is operative initsbroader phases provided ,thejmetallic ibase, dielectric `layer, Yand secondary.-e'lectron-emitting layer are as def scribed,vwithout further precautions to avoid timelag, ibut for 'best results kspecial precautions to avoid timela'g'should'b'e taken. There are several ways, which 'I`have discovered to overcome this trouble.

' One way of assisting in'fovercoming timejlag isto deposit the three-'layer secondary-electronemitting surface in suchV a waythat its temperature can `be controlled within narrow limits. Y

This can be accomplished in anumber of ways. 'Oneway is to make lthe base metal very thinso that it'has a very small heat contenty and support it ,on a heat insulator transparent to infra red radiation, such as glass, as'for example the face plate of a cathode ray tubemica, or certain plastics. The temperature is then controlled by the amount of infrared radiation'received by the composite surface. Another way is to support the very thin metallic base on asupport,

which itself has `a v'very low .heat content, suchas a Very thin nonmetallic insulating (plastic `or glass) film, which is-in turnsupportedby .a wire mesh. On being heated by asourcefof current, this wire kmesh will transmit itsheat Lto the secondary electron emitter. Still ranother way, which can be used either .byitsel-for infcombination with either infra red or electrical energy, is to make the heat contentof both the secondary-electron-emitting surface and of its .support Very low, as by supporting a very thin fmetallic base on a very thin plastic or glass iilm supported in turn on a fine plastic or-glass mesh or support, so that the heat produced by the primary beam on bombarding the surface is suicient to raise the temperature to the desired range.

It is desirable in many cases to have a'means of varying the strength of the field across the dielectric from one point on the outer surface of the secondary-electron-emitting layer to another. This can be accomplished to some-extent bythe mosaic structure of this layer, vwhich enables-different island regions of the -secondary-electronemitting layer to assume different potentials. These island regions are small in extent. The value of the differences in potential will depend on the-degree of the mosaicstructure-and on the resistivity of the dielectric, two factors determining the extent of insulation of the islands from each other. This division of the secondaryelectron-emitting composite surface can be carried even further. The surface of the-secondaryelectron-emitting layer can be divided into separate regions not only to have a means for varying the eld strength across the dielectric but for the purpose of heating the separate regions to different temperatures. Since the properties of the dielectric, such as the mobility of color centers in alkali halides, depend to a marked degree on the temperature, thisvariationii-n temperature can be used to achieve, for example, different degrees of coloration in alkali halide dielectrics. This division can be carried still further; the base metal can be divided into insulated regions which can be maintained atY different potentials.

Where the secondary-electron-emitter is beryllium, it has been successfully applied to the tube element by evaporation in vacuo from electrically heated tantalum spirals which serve as supports and heaters. It does not matter whether the beryllium is oxidized or not, since the metal and oxide are equally good as secondary emitters.

Where magnesium is used as a secondary-electron-emitter, the oxidation is essential, as the unoxidized metal is not a good emitter. The metall will, howevenjoxidize rather readily in air. I have discovered that the effectiveness of magnesium (oxidized) as a secondary-electron-emitter` vis greatly improved by the step of baking in vacuo'at a temperature of from 600 to 800 C. The reason for the improved results from the bakingin vacuo is thought to be atleast partly due to the migration of magnesium into the surface portion of the dielectric, and the fact has been clearly demonstrated.

Where the metallic base is made of a metal which forms an adherent oxide lm self limited in thickness to about 5,0 Angstroms, the oxide can be utilized for the dielectric. Suitable 'metallic base materials may Iconsist of'aluminum or aluminum alloys, such as aluminum-copper up to 11%; aluminumcopper (4 vto 4.2%) -manganese (0.5%)-magnesium (05 to 1.5%); aluminummagnesium (10%); aluminum-copper (ewa-.s

7 Silicon (3%) chromium; alloys predominantly consisting of chromium (chromium-copper) copper, alloys predominantly consisting of copper (bronze, brass, muntz metal), and corrosionresisting iron-chromium (14%) alloys.

The invention can be usefully applied to electron beam tubes and in particular to image converter, or cathode ray tubes, or light valves to intensify the images produced by electron impact on uorescent or light valve screens. .While the example given below has been directed particularly to cathode ray tubes, it will be evident to those skilled in the art that it can be applied equally well to image converters and light valves.

l For cathode ray tubes the metallic lm on the inside of the face plate must be capable of transmitting at least 85% of the light impinging upon it. It is desirable to have a means of adusting the metallic film to a definite potential. Several views show a cathode ray tube having an envelope and a cathode ray beam channel IB from a gun (Figure 8) to a metallic layer 23 on a face plate |9. For example, in Figure 2, the conducting lm 23 on the interior surface of the face plate of the cathode ray beam tube is connected toelectrode 24. The potential of the -metallic grid 25 is maintained at the value of the anode potential by connecting the grid 25 to the colloidal graphite coating 23. The grid 25 must be coated and the composite surface prepared to produce an enhanced secondary-electron-emission. The grid 25 forms the metallic base of the secondary-electron-emitter. Of course, the conducting film 23 and the electrical connection to the electrode 24 must be insulated from grid 25.

In one of the embodiments of the invention the base metal layer 23 makes electrical contact with the fine metallic wire mesh of the grid 25, the dielectric layer 2| and secondary-electronemitting layer 22 being deposited on the metallic base 20 on the side toward the electron beam. See Figure 3.

As shown in Figure 4, the fine metallic wire mesh of the grid 25 can itself supply the heat to control the temperature of the three-layer emitter. To suggest this, without limitation thereto, I have shown a heater circuit 2'| connected to an element of the mesh 25, and including a source of electric power 28 and a control resistor 29.

In another variation of the invention the threeelements secondary-electron-emitter is supported on a plastic, mica or glass grid 25 as shown in Figure 5, the electrical connection 33 being made to the base metal 2G, andthe dielectric 2| and secondary-electron-emitting layer 22 being applied thereon. Plastic for the grid may be phenolformaldehyde, linear polyamide, urea-formaldehyde, or the like.

Of the three embodiments of the invention illustrated in Figures 3, 4, and 5, especially easy to carry out in practice is the embodiment shown in Figure 5. 'Ihe base metal 20 may be varied in thickness within rather large limits depending .onthe particular application for Awhich the tube is designed. For applications involving cathode ray beam tube construction, the base metal 20 may be deposited so thinly that its transmission for light is high. v However, in many applications the power requirements of the tube, particularly of the heating means in a cathode ray beam tube, make it possible to dispense with mesh 25 provided that the secondary-electron-emitter is made part of the gatngon the face plate. Then the transparent conducting lm 23 on the interior surface of the face plate serves as the metallic base of the enhanced composite secondary electron emitter. l v

Figure 6 illustrates a tube similar to that shown in Figure 2 without a grid, in which the partially transparent conducting lm 23 provides the base for a dielectric layer 2| and a secondary-electronemitting layer 22. The terminal 24 is connected to the lm 23. As shown in the drawing, the secondary-electron-emitting layer 22 is connected electrically to an electrode 35 so that it can be maintained at a different potential from the metallic base 23. In this embodiment the potential of the secondary-electron-emitting layer 22 and of the conducting coating 26 can be the same, but if the electrodes 34 and 35 and the surfaces 22 and 26 are suitably insulated by means shown in Figure 8, this need not be so. If the potentials of surfaces 22 and 26 are the same, in many instances the electrical connection between them can be made internally without the necessity of having electrode 35.

Inorganic crystalline phosphors, inorganic vitreous phosphors, and the alkali halides are of particular interest as dielectrics over the base metal, while aluminum and magnesium, in which last metal the surface is oxidized, are of particular interest for the secondary-electron-emitting layer over the dielectric.

According to another form of realization of the apparatus according to this invention as described in the next to the last paragraph, the metallic base of the secondary-electron-emitter, which is shown as the coating 23 in Figure 2, may be divided into separate sections insulated from one another, which provide the base for the dielectric layer 2| and the secondary-electronemitting layer 22. These may also be divided into sections corresponding to the divisions of the conducting film 23. Figure 7 shows a tube in which the terminals v2| and 242 are connected electrically to the separate sections 23 and 232 of the conducting film, on which are superimposed the dielectric layer 2| and the secondaryelectron-emitting layer .22. The latter is shown as a unit insulated by the dielectric layer 2|.

In many applications, e. g. Figure 6, the potential on the partially transparent conducting nlm 23 may be negative with respect to the anode potential, but for best performance it is often advisable to maintain this conductor 23 at an alternating potential of such frequency and magnitude with respect to the anode potential that the electrons are not trapped in interstitial positions in the dielectric. This arrangement is also advantageous for the method illustrated in Figure 7 where the alternating component of the potential of the two sections of the conductor 23 and 232 should have different phase angles with respect to each other. f

Since the operation of aconstruction such as Figure 6 depends on the orientation of the electric elds, the shape of the funnel and of the face plate must be adapted to conform to the electrostatic potential lines so that the raster on the face plate should not be distorted. One way of accomplishing this is to have the end of the funnel in the shape of a cylinder with a flat face plate sealed on to the end of this cylinder. It is obvious that the funnel must have another shape if the face plate has a radius of curvaturecomparable to. its diameter. Since the nlm 23 on the interior of the face plate is anequipotential surface which may be maintained-at a .dilerent po-f tential. than the conducting coating 2t4 of the funnel, the systemV of equipotential surfaces can be made toapproximate those near theendof the low voltage cylinder of a.. two cylinder electrostatic lens. The system of mesh 2li or nlm 23 held at diierent potentialsV with` regard to the conducting anode coating 26` of the tube (aquariag isa particularly effective coating) has aprofound eiect on the electron trajectories.. In those cases where images produced by. primary beam bombardment of the enhanced secondary-electronemitting surface are under observation, the image mayl b econsiderably altered from those images which would be observed if the mesh or lm were held at, anode potential. The effects of theseV electrostatic lens systems on the electron trajectories are subject to calculation. Typical examples of this technique are shown in V. E. Cosslet, Introduction to Electron Optics (Oxford 1946) notably chapter'I II, The Electrostatic Field, where Figure 18, page 26, gives the potential distribution in the symmetrical two-.cylinder lens, and chapter III, Electrostatic Focusing, with Figure 3l, giving the trajectories and cardinal points in a two-cylinder lens. By proper choice of the potential difference theelectron trajectories become such that the spot size is substantially the same asV that in a/system Where the film 23 isA at the same potential as the conducting coating 26.

In carrying out the invention provision may have' to be madev for maintaining-without undueV leakage or breakdownf-the potential dierence in thev short'distancebetween the end of the conducting coating 2,6 at anode potential, and the conductingglm 23.- This distance has to be short in order-to` simulate the,- conditions in a typical cathode ray tube. Since thousands of volts may have to be-main-tained over this short distance ranging from 1/2Y tol inch, the glass path should be increased several times over this value. The same is true for the vacuum path between the ends of the conductingcoating 26 at anodepotential and the lm 2.3. Oneway of accomplishing this end is to form an. insulator (glass) barrier protruding` into the tubebut not so far that it intercepts the electron beam as it scans over the face plate; its simplest formv inf Figure 8V is corrugated as shown at 32vin Figure 9 like the insulators used in high tension lines, then the glass path is still furtherV increased between the end of the conducting coating'A 26 at anode potential andthe composite secondary-electron-emitting.surface in which the base metalisconstituted by thepartially transparent conducting lmmaintained at the'potential of electrodel t5l by electrical connection means. The secondary-electronfemitting llayer 22 is shown electrically insulated by the dielectric 2|;

Aefurther variationqoi the tube'.` of Figure 8 is shown in Figure 10. In this form the tubevis conveniently contouredr as'shown in Figure 8 but is provided with a nonmetallicinsulatingV (plastic or glass) lm 33-which on-itsfside-direeted toward the electron beam-1 supports the composite secondary electron'l emitting surface. TheY base metal of the three element composite surface shown in the iigure` Iasfthe partially transparent conducting film 23Y is-cennected to the terminal 35 which may be maintained ata diierentfixed potential from` theconducting coating 26, e.rg. aquadag (colloidalgr-aphite).

In order to decrease conductivity along the layer of the secondar'y-electronfemitter in the If' the barrier3l shown inr composite surfaceY such. surface may be granulated: into a mosaic. This can be doneV by firing in a hydrogen atmosphere at atemperature above 600 C. and below the softening point of the metal to reduce impurities` and roughen the surface.

An alternate technique for producing the mosaic is to expose the composite surface (metal-. lic base, oxide, and.Ysecondary-electron-emitting,

' layer) abruptly and briey to a temperature in the range fromollL to G" C..foraluminum and higher or lov/er depending on, the melting point for the other metals and alloy'smentioned. For best results the exposure `should be limited to` aY few secondszn order to insure that the secondary- .There is no exact temperature at which beading or mosaic'formation occurs, but rather a; range as set forth within which the phenomenon; is evidenced. The extent of mosaic formation is'.

influenced by SuchY factors `as oxide lmthickness and: heat capacity of the metallic base.

In` operation the semi-conductor 2| of Figures l and 2- shows metallicV conduction propertiesVy u under the action Yof the primary-electron-beam.

For.A each electron ofY the. primary beamv many electrons are raised to the conduction band of the extrinsic semi-conductor forming the dielectric 2|. In the embodiment shown in Figure 2.

'z of this application, part of these conducting band electrons` are emitted from the composite surface by the secondary-electron-emitting layer-22 and are collected by the metallic :hlm 23. In the embodiment shown in Figure 6 part of these conduction electrons arev collected by the contiguous secondary-elestron-emitter l-ayer 22 and conducted to the electrode 35. The secondary-electron-emitting layer 22v functions then as a conductor and as a collector ofonly internal secondary electrons produced inthe extrinsic semiconductor by the primary beam.

In view of my invention and disclosure variations and modication to meet individual whim or particular need will doubtless become evident' to others skilled in the art, to obtain all or part of the benets of my invention without copying the structurel shown, and I, therefore, claim all such insofar as they fall within the reasonable.

spirit and scope of my claims. Y

Having'thus described my invention, what I claim as new and desire to Patent is:

l. In anv electron'tube, a multi-plier elementV comprising a metallic base; a secondary-electron--; emitting layer andI asemi-conductor dielectric' 2. Inan electron tube, a multiplier elementv comprising a metallic base, a semi-conductor dielectric layer limited in thickness to 0.1 mmyion the base-and selected'from the class consisting of oxidesvof aluminum rcontaining impurity cenL ters, oxides of" copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors,

secure byv Letters 1 alkali halides and inorganic vitreous phosphors, and a secondary electron emitting layer from 2 to 20 microns thick on the dielectric and selected from the class consisting of beryllium, beryllium oxide, magnesium (oxididzed), oxides of magnesium base alloys, aluminum, aluminum base alloys of silicon, aluminum base alloys of magnesium, aluminum base alloys of copper, aluminum base alloys of beryllium, in which last alloy the'content of beryllium is between 25% and 40% and in which aluminum base alloy of beryllium the combined content of magnesium, molybdenum, and zirconium does not exceed two percent, oxides of aluminum base alloys having as a preconstituent beryllium, aluminum base alloys of beryllium having as a preconstituent magnesium, in which last alloy the surface is oxidized.

3. In an electron tube, a multiplier element comprising a metallic base, on the base a semiconductor dielectric layer limited in thickness to 0.1 mm., whose conductivity is articially increased under bombardment by the primary electron beam, and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a non-photoelectric secondary-electron-emitting layer on the dielectric, the positive charge in the secondary-electron-emitting layer being neutralized at a rate predetermined by the primary beam.

4. In an electron tube, a multiplier element comprising a metallic base, on the base a semiconductor dielectric layer whose conductivity is artificially increased under bombardment by the primary electron beam selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors and limited in thickness to 0.1 mm., and on the dielectric a secondary-electronemitting layer from 2 to 20 microns thick selected from the class consisting of beryllium, beryllium oxide, magnesium (oxidized), oxides of magnesium base alloys, aluminum, aluminum base alloys of copper, aluminum base alloys of beryllium, in which last alloy the content of beryllium is between 25% and 40% and in which aluminum base alloy of beryllium the combined content of magnesium, molybdenum, and zirconium does not exceed two percent, oxides of aluminum base alloys having as a preconstituent beryllium, aluminum base alloys of beryllium having as a preconstituent magnesium, in which last alloy the surface is oxidized.

5. VA cathode ray beam tube including a face plate and in close proximity to the face plate a layer, a metallic base on the layer, over the base a semi-conductor dielectric layer limited in thickness to 0.1 mm. and selected from the class consisting of the oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors,Y alkali halides, and inorganic vitreous phosphors, and over the dielectric a secondary-electron-emitting layer selected from the class consisting of beryllium, beryllium oxide, magnesium (oxidized), oxides of magnesium base alloys, aluminum, aluminum oxide, aluminum base alloys of silicon, aluminum base alloys of magnesium, aluminum base alloys of copper, aluminum base alloys of beryllium, in which last alloy the content of beryllium is between 25 and 40% and in which aluminum base alloy of beryllium the combined content of magnesium, molybdenum, and zirconium does not exceed two percent, oxides of aluminum base alloys having as a preconstituent beryllium, aluminum base alloys of beryllium having as a preconstituent magnesium, in which last alloy the surface is oxidized, and electrode means connected to the metallic base.

6. A cathode ray beam tube having a reticulated surface adapted to permit passage of primary and secondary electrons through it, a layer of dielectric upon the reticulated surface selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of magnesium, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a secondaryelectron-emitter upon the dielectric.

"7. A cathode ray beam tube having a grid, a thin partially transparent Vcontinuous layer of metal extending over the grid, and constituting the metallic base of a composite secondaryelectron-emitting surface supported thereon, a dielectric deposited upon the layer of metal, and a secondary-electron-emitting layer upon the dielectric.

8. In a vacuum tube, a metallic grid, a thinl partially transparent layer of metal supported on and in electrical contact with the grid forming the metallic base of a composite secondaryelectron-emitting surface, a dielectric deposited upon the layer of metal and a secondary-electron-emitting layer upon the dielectric.

' 9. A vacuum tube having a plastic layer, a layer of metal constituting the metallic base of a composite secondary-electron-emitting surface supported by the plastic layer, a dielectric deposited upon the layer of metal, and a seconddary-electron-emitting layer upon the dielectric.

10. A cathode ray beam tube having a face plate, means for generating an electron beam, a partially transparent non-metallic insulating layer extending across the tube between the means for generating the beam and the face plate and a secondary-electron-emitting composite surface on the partially transparent layer comprising a metallic base, a semi-conductor dielectric layer on vthe metallic base selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, the secondaryelectron-cmitting layer being located on the sides facing the means for generating the electron beam.

11. A cathode ray beam tube having a face plate, means for generating an electron beam, a transparent plastic layer extending across the tube between the means for generating the beam and the face plate and a secondary-electronemitting composite surface on the tr-ansparent layer comprising a metallic base, a semi-conductor dielectric layer on the metallic base selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, the secondaryelectron-emitting layer being located on the sides facing the means for generating the electron beam.

12. A cathode ray beam tube having a face plate, means for generating an electron beam, a transparent glass layer extending across the tube between themeans for generating the beam and the face plate anda secondary-electronemitting composite surface on the transparent glass layer comprising a metallic base, a. semiconducton' dielectric layer on the metallic base selected from the class consisting of oxides of aluminum containing impurity centers; oxides of copper, oxides of titanium, oxidesof manganese, inorganic crystalline phosphors, alkali. halides and inorganic vitreous phosphors, the secondary-electron-emitting layer being located on the sides facing the means for generating the electron beam.

13. A cathode ray beam tube having a face plate, means for generating an electron beam, a partially transparent non-metallic insulating layer extending across the tube between the means for generating the beam and the face plate` a secondary-electron-emitting composite surface on the partially transparent layer including a partially transparent metal layer as a metal base, a semi-conductor dielectric layer on the metallic base selected from the class conl sisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, the secondary-electronemitting composite surface being located on the sides facing the means for generating the electron beam and a terminal connected to such partially transparent metal layer.

14. A cathode ray beam tube having a face plate, means for generating an electron beam, a conducting coating at anode potential on the interior of the tube around the beam, a composite secondary-electron-emitting surface including as a metal b-ase a partially transparent conducting lm on the interior of the face plate and an insulator barrier extending inwardly between the conducting coating at anode potential and the partially transparent conducting lm.

15. A cathode ray beam tube having means for generating an electron beam and a face plate, a partially transparent conducting film on the interior of the face plate, constituting the metallic base of a composite secondary-electronemitting surface, a semi-conductor dielectric deposited upon the film and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a secondary-electronemitting layer upon the dielectric directed toward the electron beam.

16. A cathode ray beam tube having means for generating an electron beam and a face plate, a partially transparent conducting lm on the interior of the face plate, constituting the metallic base of a composite secondary-electronemitting surface, a` semi-conductor dielectric deposited upon the film and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, a secondary-electron-emitting layer upon the dielectric directed toward the electron beam and a terminal connected to the partially transparent conducting lm.

17. In an electron tube, a multiplier element lili having a plurality` of insulated.' metallic bases provided with separatev terminals, on and between each` base a semi-conductor dielectric layer whose conductivity isi-artificially increased under bombardment by the primary electron beam, limited in' thickness to 0.1 mm., and selected from the. class consisting of. oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors and on the dielectric a secondary-electron-emitting layer from 2 to 20 microns thick.

18. In an electron tube, a multiplier element havingv a plurality of insulated metallic bases provided with separate terminals, on and between each base a semi-conductor dielectric layer whose conductivity is articially increased under bombardment by the primary electron beam, limited in thickness toLl mm., and selectedfrom the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors on the dielectric a secondary-electron-emitting layer from 2 to 20 microns thick, and alternating current sources at different phase displacements connected to the terminals.

19. A cathode ray beam tube having means for generating an electron beam and a face plate, a partially transparent conducting lm on the interior of the 'face plate, constituting the metallic base of a composite secondary-electronernitting surface, a semi-conductor dielectric deposited upon the film and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, a secondary-electron-emitting layer upon the dielectric directed toward the electron beam, and separate terminals connected to the partially transparent conducting i'llm and to the secondary-electron-emitting layer.

20. A cathode ray beam tube having means for generating an electron beam and a face plate, a partially transparent conducting i'llm on the interior of the face plate, constituting the metallic base of a composite secondary-electronemitting surface, a semi-conductor dielectric deposited on the lm and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of magnesium, inorganic crystalline phosphors, alkali halides, and inorganic vitreous phosphors, a secondary-electron-emitting layer upon the dielectric directed toward the electron beam, and a conducting coating at anode potential on the interior of the tube around the beam.

21. An electron tube having a source of electrons, an electrically conducting metallic electrode constituting the metallic base of a composite secondary-electron-emitting surface, a semi-conductorA dielectric deposited upon the electrode, and selected from the cla-ss consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a secondary-electron-emitting layer upon the dielectric directed toward the source of electrons. I

22. An electron tube having a source of electrons, an electrically conducting metallic electrode constituting the metallic base of a composite secondary-electron-emitting surface, a semi-conductor dielectric deposited upon the electrode, and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of manganese, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a mosaic of secondary-electron-emitting material upon the dielectric directed tov/ard the source of electrons.

23. A cathode ray tube having means for generating an electron beam and a face plate, a plurality of insulated metallic bases of partially transparent conducting film on the interior of the face plate constituting the metallic bases of a, composite secondary-electron-emitting surface, a semi-conductor dielectric deposited on each base and selected from the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of magnesium, inorganic crystalline phosphors,

alkali halides, and inorganic vitreous phosphors,

a secondary-electron-emitting layer on each dielectric directed toward the electron beam, and

separate terminals connected to the partially transparent metallic bases.

24. In an electron tube, a multiplier element having a plurality of separate insulated metallic bases provided with separate terminals, on each base a semi-conductor dielectric layer of the class consisting of oxides of aluminum containing impurity centers, oxides of copper, oxides of titanium, oxides of magnesium, inorganic oxides of magnesium, inorganic crystalline phosphors, alkali halides and inorganic vitreous phosphors, and a secondary-electron-emitting layer on each dielectric.

JENNY BRAMLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,147,669 Piore Feb. 21, 1939 FOREIGN PATENTS Number Country Date 543,201 Great Britain Feb. 23, 1942 

